Some important points: there is a lot of evidence that low melatonin mediates the carcinogenic effects of circadian arrhythmia. One of the classical observations was that melatonin-depleted blood from women exposed to bright light promoted tumor growth in a model of human breast cancer and rat liver cancer (Blask et al., 2005). This is also the study that elucidated one possible mechanism: tumors take-up the n6-fatty acid linoleic acid and convert it to 13-HODE, which drives proliferation. Melatonin inhibits this (and more).
There are also data in both humans and rodents that bright light exposure during the day protects nighttime melatonin secretion against artificial light-induced suppression (eg, Kozaki et al., 2016).
This was confirmed by showing bright daytime light exposure protected sleep quality in people exposed to artificial light at night (Rangtell et al., 2016).
FYI Rodents exposed to light at night are screwed (Blask et al., 2014):
So, daytime light preserves & protects nighttime melatonin secretion. Melatonin-replete blood inhibits tumor growth. And to connect all the dots: daytime light exposure inhibits tumor growth in some pre-clinical models cancer (eg, Dauchy et al., 2015).
This isn’t a free pass for you to use artificial light at night. Ideally, you should use no artificial light at night. Or dim red ones. Or at least rock some hot blue blockers.
Nocturnal light pollution and underexposure to daytime sunlight: complementary mechanisms of circadian disruption and related diseases (Smolensky et al., 2015)
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